Japanese Published Patent Application No. 2003-51119 (Page 4, FIGS. 30 and 31) has proposed an example of an in-line circuit for an optical disc playback apparatus that plays a DVD-RAM disc as an information storage medium.
FIG. 8 is a diagram illustrating the whole construction of an optical disc playback apparatus using an in-line circuit of this type.
With reference to FIG. 8, an in-line circuit 400 is placed between an optical pickup 300 which reads information from tracks formed on a DVD-RAM disc 200 as an information storage medium, and a signal processing circuit 500 which processes a playback signal. Further, a system controller 600 controls the whole of the optical disc playback apparatus, that is, it controls the signal processing circuit 500, the rpm of a spindle motor (not shown) for rotating the DVD-RAM disc 200, tracking of the optical pickup 300, and the like.
FIG. 9 shows the specific construction of the in-line circuit 400 included in the optical disc playback apparatus. With reference to FIG. 9, reference numeral 51 denotes an input amplifier for amplifying an RF signal that is an optical playback signal from the optical pickup 300 shown in FIG. 8; numerals 52 and 53 denote external capacitors for correcting the DC level of the optical playback signal that is amplified by the input amplifier 51; numerals 54 and 55 denote switches for selecting one of the capacitors 52 and 53 according to an ID area discrimination signal 62; and numerals 56 and 57 are resistors for determining the time constant of DC level correction, in combination with the capacitors 52 and 53. Further, reference numeral 58 denotes a switch for rapidly correcting fluctuations in DC level of the optical playback signal according to a pull-in signal 63; numeral 59 denotes a buffer amplifier for amplifying the optical playback signal after the DC correction; numeral 60 denotes an AD converter for performing AD (analog-to-digital) conversion on the output signal of the buffer amplifier 59; and numeral 61 denotes a slicer as a comparator for binarizing the output signal of the AD converter 60, and outputting the result to the signal processing circuit 500 shown in FIG. 8. Further, reference numeral 62 denotes an ID area discrimination signal that turns on either of the switches 54 and 55, and numeral 63 denotes a pull-in signal that turns on/off the switch 58. The in-line circuit 400 is composed of the respective circuits placed before the AD converter 60, i.e., the input amplifier 51, the external capacitors 52 and 53, the switches 54 and 55, the resistors 56 and 57, the switch 58, and the buffer amplifier 59.
In the in-line circuit 400, the resistor 57 constitutes a high-pass filter in combination with either of the capacitors 52 and 53, and this high-pass filter removes DC components from the optical playback signal without adversely affecting group-delay characteristics of the RF signal. Usually, the cut-off frequency of the high-pass filter is set to about 100 Hz. The resistor 56 is inserted in parallel to the resistor 57 by the switch 58 in order to reduce the time constant of the high-pass filter, that is, to hasten convergence of waveform when the DC components are removed.
FIGS. 10(a)-10(d) illustrate the operation waveforms of the in-line circuit shown in FIG. 9. FIG. 10(a) shows an optical playback signal that is obtained by reading the track on the disc 200 with the optical pickup 300, which is an input signal to the input amplifier 51. In FIG. 10(a), an area where the DC level is high indicates an ID area, and an area other than the ID area indicates a data area. FIG. 10(b) shows an ID area discrimination signal 62 which becomes “High” when a portion of the track corresponding to the ID area is played, thereby to turn on the switch 54 and off the switch 55. At other times, the ID area discrimination signal 62 turns on the switch 55 and off the switch 54. FIG. 10(c) shows a pull-in signal which becomes “High” for a predetermined period from a playback area switching point, i.e., a point in time when the playback area is switched between the data area and the ID area, thereby to turn on the switch 58. FIG. 10(d) shows an input signal to the AD converter 60.
FIG. 11 is a diagram for explaining the structure of the ID section of the DVD-RAM disc, wherein 100 denotes marks which are recorded in grooves (guide grooves) and in a land sandwiched between the grooves, and 101 denotes embossed marks formed in the ID area.
As shown in FIG. 11, since the data area of the track is of the land-groove structure, even a portion with no mark 100 does not have the maximum reflectivity. However, since the ID area is of the embossed structure, a portion with no embossed mark 101 has mirror reflectivity. Therefore, as shown in FIG. 10(a), the DC level of the playback signal in the data area is significantly different from that in the ID area.
By the way, the input range of the AD converter 60 is set in accordance with the amplitude of the RF component of the playback signal as shown in FIG. 10(a), and it is smaller than the amplitude including the above-mentioned DC level fluctuations. So, it is necessary to perform AD conversion while suppressing the DC level fluctuations.
Therefore, as shown in FIG. 9, the data area capacitor 52 and the ID area capacitor 53 are used in combination as coupling capacitors, and switching between the capacitors 52 and 53 is performed by controlling the switches 54 and 55 with the ID area discrimination signal 62, thereby generating a signal having a uniform DC level as shown in FIG. 10(d).
Thereafter, in order to speedily carry out charging of the DC voltage to the data area and the ID area, a resistor 56 against the capacitors 52 and 53 is inserted in parallel to the resistor 57 by the switch 58, whereby the time constant is reduced, and convergence of the playback signal at the ID area start point and end point is speeded up.
As described above, in the in-line circuit of the conventional optical disc playback apparatus, when a track formed on an optical disc is played, it is necessary to switch between the coupling capacitors 52 and 53 according to whether the area being played is an ID area or a data area. In order to carry out this switching, a circuit for generating an ID area discrimination signal is needed besides the in-line circuit.
FIG. 12 is a block diagram illustrating a circuit for generating an ID area discrimination signal, and FIG. 13 shows the operation waveform thereof.
In FIG. 12, reference numeral 80 denotes a photoreceptor of the optical pickup 300 shown in FIG. 8, and the photoreceptor 80 is divided into four parts with respect to the track direction of the optical disc. Reference numerals 81 and 82 denote adders, and numeral 83 denotes a subtracter. The adders 81 and 82 calculate the sum of the outputs from the photoreceptors A and D and the sum of the outputs from the photoreceptors B and C, respectively, and the subtracter 83 subtracts the output signal of the adder 82 from the output signal of the adder 81, thereby generating a differential RF signal. Reference numeral 84 denotes a buffer amplifier for amplifying the differential RF signal. Reference numerals 85 and 86 denote a resistor and a capacitor which constitute a high-pass filter, respectively, and the high-pass filter removes DC components from the differential RF signal outputted from the buffer amplifier 84, thereby assisting the subsequent-stage processing using threshold values. Reference numeral 87 denotes a buffer amplifier for amplifying the output of the high-pass filter. Reference numerals 90 and 91 denote comparators for comparing the differential RF signal outputted from the buffer amplifier 87 with an upper threshold value 88 and a lower threshold value 89, respectively. Reference numeral 92 denotes an OR circuit for generating an OR of the output signals of the comparators 90 and 91, and numeral 93 denotes a monostable multi vibrator (hereinafter referred to as a mono multi circuit) for removing RF frequencies from the output signal of the OR circuit 92, thereby to waveform-shape the signal into continuous binary signals.
Further, FIG. 13(a) shows the differential RF signal in the data area and the ID area, and the levels of the upper threshold value 88 and lower threshold value 89. FIG. 13(b) shows the output of the upper comparator 90, FIG. 13(c) shows the output of the lower comparator 91, and FIG. 13(d) shows the output of the mono multi circuit 93.
As shown in FIG. 11, each embossed mark 101 in the ID area of the DVD-RAM disc is formed in a position that is ½ track offset from the center of the data track constituted by the lands and the grooves, and therefore, the differential RF signal in the ID area is offset upward and downward with respect to the RF signal in the data area as shown in FIG. 13(a). The ID area can be detected by detecting this offset. Although, in the example shown in FIG. 11, the differential RF signal is offset upward in the first half of the ID area and downward in the second half of the ID area, the reverse may occur. In the input signal to the in-line circuit shown in FIG. 9, the whole ID area is offset upward with respect to the data area, as shown in FIG. 10(a). This is because the in-line circuit receives not the differential RF signal but the total sum of the output signals from the four photoreceptors of the optical pickup 4.
In the ID area discrimination signal generation circuit shown in FIG. 12, the comparators 90 and 91 compare the waveform shown in FIG. 13(a) with the upper threshold value 88 and the lower threshold value 89, thereby detecting the upper and lower offsets, respectively. The signals thus obtained are shown in FIGS. 13(b) and 13(c), and these signals are unified and shaped to generate an ID area discrimination signal shown in FIG. 13(d).
In recent years, cost reduction has rapidly proceeded for the optical disc playback apparatus, and it is urgently needed to streamline the circuits in the apparatus in response to the cost reduction.
As for the in-line circuit, it is difficult to incorporate the coupling capacitors into a LSI, and therefore, external capacitors should be provided. Thereby, the number of terminals of the LSI increases, and a special circuit for generating a timing signal (switching signal) such as an ID area discrimination signal is required to realize in-line control, leading to an increase in cost for DVD-RAM playback.
As a countermeasure against the above-mentioned problem, it is considered that the playback signal from the optical disc is AD-converted and then inputted to a high-pass filter having a cut-off frequency of 100 Hz, thereby to dispense with the coupling capacitors. However, since the optical disc playback signal includes DC level fluctuations as described above and therefore its input dynamic range is large, a high-performance AD converter is required for AD-converting the playback signal, which conversely causes an increase in cost.